Why is this guy so secretive?

I really like this guy, but he seems so secretive and it makes me wonder whether I said or did something that made him secretive towards me, or if that's just his personality. Like when I ask how his day went, he would say stuff like busy but great, but he wouldn't elaborate on it. Whereas I tend to be the complete opposite. Life I don't share my life story, but I like being more open towards who I am whereas he seems to hold it in for some reason.